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- 1. Metallurgy & Material Science<br />Dr.S.Jose<br />Dept of Mechanical Engg.,<br />TKM College of Engineering, Kollam<br />
- 2. 2<br />Module II<br /><ul><li>Diffusion in crystals
- 3. Theory of Alloys
- 4. Equilibrium Diagram
- 5. TTT Diagram
- 6. Heat Treatment
- 7. Recovery, Recrystallisation & Grain Growth</li></li></ul><li>Diffusion<br />What is Diffusion <br />Mechanisms of diffusion<br />Vacancy<br />Interstitial<br />Steady-state diffusion<br />Non-steady-state diffusion<br />Fick’s laws<br />
- 8. What is Diffusion<br />The phenomenon of material transport by atomic motion.<br />Diffusion couple Cu&Ni is heated to high temperature and cooled.<br />Cu atoms migrated or diffused into Ni and Ni atoms into Cu.<br />The process in which atoms of one metal migrate into another - Inter-diffusion or impurity diffusion<br />Diffusion occurring in pure metals –atoms exchanging positions- called self diffusion<br />
- 9. Mechanisms of diffusion<br />Diffusion is a stepwise migration of atoms from lattice site to lattice site.<br />For an atom to make such a move,<br />There must be an empty adjacent site.<br />The atom must have sufficient energy to break bonds with its neighboring atom.<br />Vacancy diffusion<br />Interchange of an atom with an adjacent vacant lattice site. (self- and inter-diffusion)<br />Interstitial diffusion<br />Migration of atoms from an interstitial position to a neighboring empty one.<br />
- 10. Mechanisms of diffusion<br />
- 11. Diffusion – time function?<br />Steady-state and Non-steady-state diffusion processes are distinguished by the parameter – diffusion flux, J.<br />Flux is defined as number of atoms crossing a unit area perpendicular to a given direction per unit time.<br />Flux has units of atoms/m2.sec or moles/m2.sec.<br />If the flux is independent of time, then the diffusion process is called steady-state diffusion. <br />On the other hand, for non-steady-state diffusion process, flux is dependent on time.<br />
- 12. Fick’s first law<br />States that diffusion flux is proportional to concentration gradient<br /> is the gradient of the concentration C<br /><ul><li>The proportionality constant, D, is called diffusion coefficient or diffusivity. It has units as m2/sec.
- 13. The diffusion coefficient of a material is also referred to as 'diffusion constant’.
- 14. The negative sign of the right side of the equation indicates that the impurities are flowing in the direction of lower concentration.</li></li></ul><li>Fick’s second law <br />Fick's First Law does not consider the fact that the gradient and local concentration of the impurities in a material decreases with an increase in time, an aspect that is important to diffusion processes.<br />The Second Law states that the change in impurity concentration over time is equal to the change in local diffusion flux<br />is the rate of change of concentration with respect to time at a particular position, x.<br />
- 15. Theory of alloys<br />What is an alloy<br />Types of alloys<br />Solid solution<br />Substitutional<br />Interstitial<br />Intermetallic compounds<br />Hume – Rothery Rules<br />Superalloys<br />
- 16. Alloys<br />An alloy is a phase comprising of one or more components.<br />There are three distinguishable types:<br />Substitutional: solute substitutes the solvent in the crystal lattice without structural changes.<br />Interstitial: solute does not occupy the sites in the lattice of the solvent, but resides in crystallographic pores.<br />Transformational: A completely new lattice is formed. Usually occurs as a result of intermetallic compound formation.<br />
- 17. Substitutional Solid Solution<br />In substitutional solid solution, the arrangement of the solute atoms may be disordered (random) or ordered.<br />Some alloy systems exhibit complete solid solubility (e.g. Cu-Ni, Cd-Mg), others show only limited solubility at any temperature.<br />Several factors determine the limits of solubility. <br />These are expressed as a series of rules often called Hume-Rothery Rules.<br />
- 18. Substitutional Solid Solution <br />
- 19. Hume-Rothery Rules<br />The Hume-Rothery rules are a set of basic rules describing the conditions under which an element could dissolve in a metal, forming a solid solution. <br />There are two sets of rules, one which refers to substitutional solid solutions, and another which refers to interstitial solid solutions.<br />
- 20. Hume-Rothery Rules<br />Hume-Rothery Rule 1:Atomic Size Factor Rule. <br />The atomic radii of the solute and solvent atoms must differ by no more than 15%: <br />Extensive substitutional solid solution occurs only if the relative difference between the atomic diameters (radii) of the two species is less than 15%. If the difference >15%, the solubility is limited.<br />
- 21. Hume-Rothery Rules<br />Hume-Rothery Rule 2:Crystal Structure Rule .<br />The crystal structures of solute and solvent must match.<br />For appreciable solid solubility, the crystal structures of the two elements must be identical.<br />Hume-Rothery Rule 3: Valency Rule .<br />Maximum solubility occurs when the solvent and solute have the same valency. Metals with lower valency will tend to dissolve metals with higher valency. <br />A metal will dissolve a metal of higher valency to a greater extent than one of lower valency. <br />
- 22. Hume-Rothery Rules<br />Hume-Rothery Rule 4: The Electronegativity Rule <br />The solute and solvent should have similar electronegativity. <br />If the electronegativity difference is too great, the metals will tend to form intermetallic compounds instead of solid solutions. <br />
- 23. Interstitial solid solution<br />Interstitial solid solutions are formed if<br />a solute is smaller than the pores in the lattice of a solvent<br />a solute has approximately the same electronegativity as a solvent.<br />There are very few elements that create ions, small enough to fit in interstitial positions, therefore, appreciable solubility is rare for interstitial solid solutions.<br />Ions that often may be a solute in solid solutions are: H, Li, Na, B. <br />
- 24. Hume-Rothery Rules<br />For interstitial solid solutions, the Hume-Rothery rules are:<br />Hume-Rothery Rule 1:<br />Solute atoms must be smaller than the pores in the solvent lattice. <br />Hume-Rothery Rule 2: <br />The solute and solvent should have similar electronegativity.<br />
- 25. Intermetallic compounds<br />Ordered intermetallic compounds are formed if the components have very different electronegativity.<br />
- 26. Classification scheme for alloys.<br />
- 27. Superalloys<br />Superalloys is a name for a group of alloys that retain high strength at elevated temperatures.<br />The main strengthening mechanism is preventing grain boundaries from sliding via dislocation.<br />Most of the superalloys are Substitutional solutions, where one of the components tends to form covalent bonds (Al in Ni3Al, W in Ta or Nb.)<br />
- 28. 23<br />Module II<br /><ul><li>Diffusion in crystals
- 29. Theory of Alloys
- 30. Equilibrium Diagram
- 31. TTT Diagram
- 32. Heat Treatment
- 33. Recovery, Recrystallisation & Grain Growth</li>

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